Solar-Hydrogen Energy Systems (eBook)

An Authoritative Review of Water-Splitting Systems by Solar Beam and Solar Heat: Hydrogen Production, Storage and Utilisation

Tokio Ohta (Herausgeber)

eBook Download: PDF | EPUB

2013 | 1. Auflage
276 Seiten
Elsevier Science (Verlag)
978-1-4831-8842-3 (ISBN)

Infos

Solar-Hydrogen Energy Systems is a collection of papers that discusses the advancements in the research of alternative energy technologies that utilizes solar-hydrogen energy systems. The text first introduces the concept of solar-hydrogen energy system, and then proceeds to covering the technical topics in the subsequent chapters. The next chapters talks about the thermodynamics of water-splitting and water electrolysis. Next, the selection details direct thermal decomposition of water. The selection also discusses different processes to produce hydrogen, such as thermochemical, photochemical, and biochemical. The ninth chapter talks about solar energy storage by metal hydride, and the last chapter deals with direct solar energy conversion at sea. The book will be of great interest to scientists, engineers, and technicians involved in the research, development, and implementation of alternative energy technology.

Textauszug

CHAPTER 2

THERMODYNAMICS OF WATER-SPLITTING

Publisher Summary

This chapter explains the thermodynamics of water splitting. An enormous quantity of water exists on the earth’s surface; it is estimated to be about 13.8 108 km3. About 97% of water is in the oceans and about 2% is in the perpetual ice and snow of the polar regions. The precipitation in a year is estimated to be about 105 knr3, 60% of which is vaporized. A part of the remainder goes to the ocean and the rest is absorbed by the earth or living things. The water that is contained by all the living species on the earth is estimated to be about 2 x 1012 bbl, which is more than the total petroleum reserves in the world. Although water is a common substance, the quantitative understanding of the thermodynamical properties of water, that is, heat of vaporization, melting, and boiling temperatures, specific heat, etc., requires a new concept of hydrogen bonding.

2-1 STRUCTURE AND DYNAMICS OF WATER MOLECULE

An enormous quantity of water exists on the earth’s surface, it is estimated to be about 13.8 ×108 km3. Ladling it by a cubic measure of 1 km3 and lining them up between the earth and the sun, more than four returns are possible. About 97% of it are in the oceans and about 2% are in the perpetual ice and snow of the polar regions. The precipitation in a year is estimated to be about 105 km3, 60% of which is vaporized. A part of the remainder goes to the ocean and the rest is absorbed by the earth or living things. The water which is contained by all the living species on the earth is estimated to be about 2 ×1012 bbl, which is more than the total petroleum reserves in the world.

These enormous amount of water on the earth is not of single kind, because the structure of only pure water (gas, liquid and solid phases) has been studied and most of existing water is not pure, dissolving many kinds of materials. Sea water, the most plentiful material in the world, contains a lot of chemical elements, some of which are listed in Table 2.1. Pure water occurs very rarely in nature. The data of ion dissociation in pure water are ahown in Table 2.2 [1].

Table 2.1

Main components of sea water

Table 2.2

Data of ion dissociation in pure water at 25°C

Although water is a common substance, quantitative understanding of the thermodynamical properties of water, i. e., heat of vaporization, melting and boiling temperatures, specific heat, etc., requires a new concept of hydrogen bonding. For example, the heat of vaporization of water is more than twice that of H2S which belongs to the same family of hydrogen compounds. This shows an extraordinary strength of the cohesive force that can be explained by hydrogen bonding. The hydrogen bond in water is described as O–H …, where the “O–H” is a proton donor and the “O” acts as a proton acceptor. The hydrogen atom in O–H is positively charged on account of the strong electronegativity of the combined oxygen atom, while the oxygen atom connected by the dotted line in O–H … O is negatively charged. Thus the hydroxyl group O–H and the oxygen atom will interact with each other by the attractive electrostatic force. This is one origin of the hydrogen bond.

Another origin is a covalent bonding between the H in O–H and the O in O–H…O. The force range of the hydrogen bond is longer than that of van der Waal’s force which originates from the dipole moments of each water molecules.

A model of water due to Walrafen [2] in a liquid state is shown in Fig. 2.1. The binding of each water molecule is hydrogen bond which is shown by a series of small circular plates. However, examination of water in liquid state using X-ray diffraction method has led to precise knowledge about the molecular configuration.

Fig. 2.1 Structure of water by hydrogen bonds. Small sphere and white big sphere represent the hydrogen and oxygen atoms, respectively. Hydrogen bond is shown by circular plates [2].

According to them, some models for the configuration of the water molecules have been proposed. An example is the interstitial model (or intermingled model) which describes a framework formed by the first kind of molecule interacting with hydrogen bonds, and individual water molecules belonging to the second kind of molecules existing in the vacancies of that framework (Samilov, 1965). Besides, a couple of models have been proposed, e.g., Bernal, 1964). All of them state that the structure of water in the liquid state has a short range order and is bonded partially by hydrogen bondings. The structure of ice is understood better than liquid water. Ice has a long range order and is bonded by hydrogen bondings.

For an individual molecule of water, the electrostatic model as shown in Fig. 2.2 is usually pictured. This is convenient for describing the electric field around the molecule. An oxygen atom is located at the site noted by +6e (positive charge six times of the proton) and two hydrogen atoms are placed at the two sites noted by +(1/2)e. The distance between the oxygen and the hydrogen atoms is 0.99 A and the bond angle between the two O–H bonds is 105°3’. The negative charge of −7e which neutralizes the molecule is located at 0.024 A from the oxygen molecules on the bisector of the bond angle.

Fig. 2.2 Electrostatic point charge model for a water molecule.

From Fig. 2.2, one can readily understand that the water molecule has a dipole moment (μ) and a quadropole moment (q). The magnitudes of them are μ = 1.84 × 10−18 esu · cm2 and q = −5.6 × 10−26 esu · cm2, respectively. The dipole-dipole interaction is the main origin of the intermolecular van der Waals’ force and takes place also in liquid water. Intermolecular force in water is due to a subtle combination of van der Waals’ force with hydrogen bond, which builds up the special nature of water in scientific sense, though common in daily experience.

Vibration of a water molecule has three normal modes that, are denoted by v1/2, v2 and v3. All other vibration modes can be composed by these three normal modes. As are shown in Fig.2.3, v1 and v3 are the expansion and contraction motion of the hydrogen atom along the O–H bond, while v2 is the motion of the hydrogen atom perpendicular to the O–H bond and is called deformation vibration. The first excited state of v2 mode occurs from the ground state by absorbing infra-red beam, wave number of which is 1, 594.59 cm−1. This excitation occurs easily. The first excitation of v1 and v3 are done by infra-red beams with 3, 656.65 cm−1 and 3, 755.79 cm−1, respectively. Even at the temperature of the absolute zero, the zero point vibration takes place. The higher the temperature becomes, the more violent the vibration. Not only the vibrational motion but also the rotational motion is excited and every bond will be cut off at the vaporizing point. If the temperature rises higher and higher, then the bond between the oxygen and the hydrogen will be broken, bringing about the dissociation of water molecule into hydrogen and oxygen.

Fig. 2.3 Normal vibrations of water molecule.

2-2 THERMODYNAMIC PARAMETERS

Let us firstly discuss the heat of formation whose absolute value is equal to the energy needed to split water. From the measurements of combustion heat and spectroscopic dissociation heat, the following numerical values are known.

For the vapor at 297°C and 1 atm, we have

(2.1)

For the liquid, these are

(2.2)

In Eqs. (2.1) and Eqs. (2.2), the thermodynamic parameters Ho (enthalpy), Go (Gibbs’ free energy) and So (entropy) represent their standard values at 298 k and 1 atm which are written sometimes as Ho298, etc.˙

The vapor dissociation has four steps and the energies ΔE for those processes are theoretically given by

(2.3)

(2.4)

(2.5)

(2.6)

The net change of the total energy is 57.10 kcal/mol, which is slightly different from the empirical value of ΔHo in Eq. (2.1). This is mainly due to the correction of volume change that is neglected in calculating ΔE.

Throughout this book as well as in general, we use the values described in Eqs. (2.1) and Eqs. (2.2). The thermodynamic parameters H, G and S are functions of temperature and pressure.

Next, we consider thermodynamically the water decomposition process shown by Eq. (2.2) The internal energy and the volume of this system in the state of 25°C and 1 atm are denoted by U and V, respectively. If this system is subjects to a change, accordingly U and V change to U + ΔU and V + ΔV at 25°C and 1 atm, then the change of enthalpy is

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Produkt-Details

• Erscheint lt. Verlag: 22.10.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 1-4831-8842-6 / 1483188426
• ISBN-13: 978-1-4831-8842-3 / 9781483188423